Uncertainties in Predictions by Complex System Codes

Uncertainty analysis aims at characterizing the errors associated with experiments and predictions of computer codes, in contradistinction with sensitivity analysis, which aims at determining the rate of change (i.e., derivative) in the predictions of codes when one or more (typically uncertain) input parameters varies within its range of interest. In the present paper the salient features of established approaches for estimating uncertainties associated with predictions of complex system codes are reviewed together with the reasons why uncertainty evaluation is mandatory in nuclear reactor safety and licensing.